This invention relates to electro-optical techniques for making accurate measurements of objects at high speeds without physically touching them. It can be used, for example, to measure objects while they are moving along a factory production line at an uncertain distance with respect to a remote electro-optical measuring system, and it makes feasible object measurements at speeds and accuracies that were heretofore unattainable.
A technique in accordance with the invention involves illuminating the object with a controlled field of radiant energy and then scanning the image which results from the interaction of the object and the field by means of an electro-optical system which includes a mini-computer. The combination of a controlled field and a scanner system yields a range of performance capabilities which has significant practical advantages.
There are many examples in the prior art wherein controlled light fields are used in instruments designed for visual observation and for electro-optical measurement. Thus, microscope substage illuminators are usually designed to deliver light flux through a controlled area corresponding to the field of view, with a controlled degree of angular divergence corresponding to the numerical aperture of the viewing system.
The significance of divergence control in microscope illumination on the acuity of visual observation has been known for over 100 years. Consistent with this knowledge, modern microscope illuminators are typically designed to generate a large angular divergence of illumination in order to produce high visual resolution. In this regard reference is made to "Technical Optics" by L. C. Martin, published by Pittman & Sons Ltd., 1954 (Volume II, Chapter III, titled "The Microscope"--pages 80-146).
With the advent of laser technology and coherent holography, renewed interest has been aroused in the study of a controlled light field. But the purpose of this study has largely been to improve the resolving power of instruments for visual imaging under laboratory conditions, rather than to develop electro-optical instruments which yield precise object measurements at high speeds in industial environments.
Another application of controlled light fields in optical instrumentation has been the occasional use of collimated illumination in optical comparators in order to maintain the apparent size of the visual image approximately constant for small longitudinal displacements of the object under examination. This type of controlled light field, designated "telecentric illumination" has been of limited utility because it increases image blurring due to diffractive effects and thereby further compromises an already limited precision of dimensional readout due to the limitations of human visual contrast perception. In this regard, reference is made to "Applied Optics and Optical Engineering" by R. Kingslake--Academic Press--1965 (Vol. I, Section 6 III. 4, page 232, entitled "Limitations of Ray Bundles--Telecentric Systems").
There appears, however, to be no prior appreciation of the high accuracy of object measurement plus the great freedom of object motion which are attainable by using a controlled illumination field in conjunction with a properly programmed electro-optical scanner.
In the copending application Ser. No. 374,113, noncontact dimensional measurement of objects is effected by an electronic caliper technique wherein an electronic scan acts to determine the distance between opposite edges of the object in a manner analogous to that of a mechanical caliper whose jaws engage the opposite edges. In the system disclosed in this copending application, an outline or silhouette of the object is optically projected onto the sensitive face of an electro-optical scanner. A scan generator coupled to the scanner develops scan voltages to create a scan path which traverses those edges of the image of the object outline which constitute the terminal points of a dimension to be measured.
In the later filed copending application Ser. No. 504,289, which is a continuation-in-part of the earlier-filed co-pending application, use is made of a coordinatograph technique in which the scanner is directed to look only in the vicinity of the edges whose coordinates are to be determined. A difficulty experienced with the techniques disclosed in these copending applications arises in those situations where a part or object being measured is moving toward or away from the scanner, for then the image thereof changes in scale and the trace does not reflect the true dimensions of the part.